A conventional television tube comprises an almost plane faceplate or screen of rectangular shape. The screen is furnished on its internal face with a mosaic of patches of phosphors or pixels which excited by an electron beam emit light which may be blue, green or red, depending on the phosphor excited.
An electron gun sealed in the envelope of the tube is directed towards the centre of the screen and makes it possible to emit the electron beam towards the various points of the screen through a perforated mask (or shadow mask). The electron gun makes it possible to focus the electron beam onto the internal face of the screen carrying the phosphors.
A deviating system placed around or on either side of the tube makes it possible to act on the direction of the electron beam so as to deviate its trajectory. Continual action of the deviating system thus allows horizontal and vertical scanning of the screen so as to explore the entire mosaic of phosphors.
Without deviation of the electron beam and with symmetric electrodes of the gun that create symmetric electric fields in the gun, the electron beam reaches the centre of the screen and the spot formed is circular.
When the deviating system is acted on and the direction of the beam is deflected, the spot on the screen is deformed and the problem is all the more crucial as the beam is deflected towards the periphery of the screen or even towards the corners of the screen. In particular, in the case of a rectangular screen whose large dimension is horizontal, a horizontal deflection towards the left and right edges gives rise to a horizontally deformed spot. In the corners there is a vertically and horizontally combined deformation.
To remedy these defects, the art makes provision for electrodes made in the form of quadripoles and controlled electrically in different ways in the vertical direction and in the horizontal direction, doing so in order to precompensate for the deformations of the beam just described.
The quadripolar effects thus make it possible to achieve shape factors for the electron beams. These effects tend to counter the phenomena of distortion of shapes of beams created by the deviator in a situation of deviation towards the periphery of the screen and hence of deformation of size of spot on the screen. The shape factor must be dynamic as a function of the deviation of the beam.
The horizontal distortion of the electron beam towards the periphery of the screen is therefore the result of a magnetic deflection caused by the deviator deflecting the beam so as to effect the scanning of the screen, and associated with this deviator the action of an exit quadripole in the gun. The combining of these effects results in a degradation of the horizontal resolution and a large improvement in the vertical resolution.
As is represented in FIG. 2a, the electric lines of force are therefore oriented in the direction of the arrows 4 and the electron beam undergoes a force of compression 2 in the horizontal plane and of distortion 3 in the vertical plane. Starting from the principle that an electron beam must preferably occupy a large enough surface area in the plane of a principal exit lens of an electron gun to obtain a best possible resolution and that the size of the beam at the screen edge is not optimal for good resolution, a problem to be solved arises.
Among the various possibilities of structures, a quadripole structure using three electrodes is used. Such is the case for example of the structure described in patent U.S. Pat. No. 5,027,043. In this system, the entrance and exit electrodes are at a variable potential, this giving rise to an alteration of the optical properties of the lenses upstream and downstream of this system. The object of the invention is therefore to improve the shape of the spot at the screen edge on a high-definition television screen and in particular to increase the horizontal dimension of the electron beam in the principal lens of the gun. To do this, the invention makes provision to fit a quadripolar device making it possible to cancel the undesirable effects produced by the quadripolar device fitted at the exit of the gun and by the deviator.